pubsub: add EXPERIMENTAL notice to ExpirationPolicy
pubsub: add EXPERIMENTAL notice to ExpirationPolicy

Post-merge updates suggested by Jean, from CL 40772.

Change-Id: Ia41af0a129fc28b86d007ff39496c89fec8bb137
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40890
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go